A. Image Projection Systems
Image projection systems are used to form an image of an object, such as a display panel, on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
FIG. 1 shows in simplified form the basic components of an image projection system 77 for use with a microdisplay imaging device (also known in the art as a “digital light valve” or a “pixelized imaging device”). In this figure, 70 is an illumination system, which comprises a light source 71 and illumination optics 72 which transfer some of the light from the light source towards the screen, 73 is the imaging device, and 74 is a projection lens which forms an enlarged image of the imaging device on viewing screen 75.
For ease of presentation, FIG. 1 shows the components of the system in a linear arrangement. For a reflective imaging device of the type with which the present invention is concerned, the illumination system will be arranged so that light from that system reflects off of the imaging device, i.e., the light impinges on the front of the imaging device as opposed to the back of the device as shown in FIG. 1. Also, as shown in FIGS. 2 and 3, for a reflective imaging device which operates by modulating (changing) the polarization of portions of the illumination light (referred to herein as a “reflective, polarization-modulating, imaging device”), a polarization beam splitter (PBS) will be located in front of the imaging device and will receive illumination light 11, e.g., S-polarized light, from the illumination system and will provide imaging light 12, e.g., P-polarized light, to the projection lens.
For front projection systems, the viewer will be on the left side of screen 75 in FIG. 1, while for rear projection systems, the viewer will be on the right side of the screen. For rear projection systems housed in a cabinet, one or more mirrors are often used between the projection lens and the screen to fold the optical path and thus reduce the system's overall size.
Image projection systems preferably employ a single projection lens which forms an image of: (1) a single imaging device which produces, either sequentially or simultaneously, the red, green, and blue components of the final image; or (2) three imaging devices, one for red light, a second for green light, and a third for blue light. Rather than using one or three imaging devices, some image projection systems have used two or up to six imagers. Also, for certain applications, e.g., large image rear projection systems, multiple projection lenses are used, with each lens and its associated imaging device(s) producing a portion of the overall image.
B. Polarization Beam Splitters
FIG. 2 shows a conventional layout for an image projection system employing a polarization beam splitter 60 of the MacNeille cube type. See, for example, E. Stupp and M. Brennesholtz, “Reflective polarizer technology,” Projection Displays, 1999, p. 129-133. As shown in this figure, the polarization beam splitter (PBS) consists of two optically cemented right-angle prisms 61 and 62. The diagonal 63 of the splitter has a dielectric coating that reflects S-polarized light and transmits P-polarized light.
As can be seen in FIG. 2, after reflecting off of the diagonal of the MacNeille-type PBS, S-polarized light 14 from the illumination system reaches reflective imaging device 10, e.g., a LCoS device, where it is polarization modulated. The modulated light 15 is P-polarized and thus passes through diagonal 63 and on to the projection lens to form the desired image. Non-modulated light (not shown in FIG. 2), which is still S-polarized, reflects from the diagonal and is returned to the illumination system.
The main problem with using a MacNeille-type PBS in image projection systems is the depolarization of transmitted light that is caused by skew-ray effects. This is a purely geometrical phenomenon and is described in Miyatake, U.S. Pat. No. 5,327,270, which issued on Jul. 5, 1994, and is entitled “Polarizing Beam Splitter Apparatus and Light Valve Image Projection System.”
This depolarized light reduces the contrast of the system. In accordance with the Miyatake patent, compensation of the skew-ray depolarization requires an additional quarter-wave plate (i.e., plate 64 in FIG. 2), which adds cost, requires precision alignment, and restricts the range of operating temperatures. Other disclosures of the use of compensating plates in projection systems employing reflective, polarization-modulating, imaging devices can be found in Ootaki, U.S. Pat. No. 5,459,593, Schmidt et al., U.S. Pat. No. 5,576,854, and Bryars, U.S. Pat. No. 5,986,815.
Another type of PBS is the wire grid polarizer. See, for example, Perkins et al., U.S. Pat. No. 6,122,103, which issued on Sep. 19, 2000, and is entitled “Broadband Wire Grid Polarizer for Visible Spectrum.” This optical component does not suffer from skew-ray depolarization, and also has a very high polarization extinguish ratio. In addition, the component works over a large temperature range and can withstand a high light intensity. A wire grid polarizer 13a can be used with reflective, polarization-modulating, imaging devices, e.g., LCoS devices, in accordance with the component layouts shown schematically in FIGS. 3A and 3B. Unfortunately, both of these layouts suffer from optical problems.
The optical problem associated with the layout of FIG. 3A is that there is a tilted plano-parallel plate in the imaging optical path. The plate is the glass substrate (thickness greater than 0.5 mm) that supports the wire grid structure. Currently, a technological limitation in the process that creates the wire grid structure makes the use of a thinner substrate difficult. A glass substrate 0.5 mm thick, tilted at 45 degrees creates astigmatism of −0.135 mm. See Warren J. Smith, Modern Optical Engineering, 2nd edition, McGraw-Hill, Inc., New York, 1990, page 99. The typical depth of focus of a projection lens used with a LCoS device is +/−0.025 mm (for an f-number (FNo) of 2.8). Therefore, the layout of FIG. 3A has unacceptable image quality due to astigmatism that is 2.5-3 times larger than the depth of focus.
In the layout of FIG. 3B, the light passes through the tilted glass substrate in the illumination path, where astigmatism is not critical. In this case, the image quality in the imaging path depends on the flatness of the wire grid substrate. Acceptable image quality requires the surface flatness to be about 1 fringe per inch or better. The best wire grid polarizers available today have a flatness of about 3 fringes per inch. There are also two other problems associated with the layout of FIG. 3B: (1) temperature deformation and (2) wire grid structure protection.
Typically, a LCoS projector is assembled and aligned at room temperature, but the operational temperature in the area of the LCoS (where the wire grid PBS is located) is 45-55 degrees Celsius. This elevated temperature can create deformation of the wire grid substrate, which will degrade the image quality on the screen.
As to the protection problem, the wire grid structure should be protected from environmental dust, humidity, mechanical scratches, etc., which will reduce the polarization properties of the PBS. But any kind of protective window applied in front of the grid structure in the configuration of FIG. 3B will essentially reintroduce a plano-parallel plate into the imaging path, which will create astigmatism as discussed above.
Another known type of PBS is a multi-layer reflective polarizer. See, for example, Jonza, et al., U.S. Pat. No. 5,965,247, the contents of which are incorporated herein by reference. See also Private Line Report on Projection Display, Volume 7, No. 11, Jul. 20, 2001, pages 6-8.
Like wire grid polarizers, multi-layer reflective polarizers fall into the general class of Cartesian polarizers in that the polarization of the separate beams is referenced to the invariant, generally orthogonal, principal axes of the polarizer so that, in contrast with a MacNeille-type PBS, the polarization of the separate beams is substantially independent of the angle of incidence of the beams. See Bruzzone et al., U.S. Pat. No. 6,486,997, the contents of which are incorporated herein by reference.
FIG. 3C schematically shows a layout for using a multi-layer reflective polarizer 13b with a reflective, polarization-modulating, imaging device 10, e.g., a LCoS device. Multi-layer reflective polarizers are relatively thick components and, as shown in FIG. 3C, are tilted at an angle of 45 degrees.
The thickness of this component in combination with the difference in refractive index between the component and the surrounding glass prisms 51 and 52 creates astigmatism, which degrades the display's image quality. For example, a multi-layer reflective polarizer can have a thickness and index of refraction of 0.25 millimeters and 1.54, respectively, while the index of refraction of prisms 51 and 52, when composed of PBH-56 glass, is approximately 1.85. When tilted at 45°, such an arrangement creates astigmatism of approximately 0.2 mm. To correct this astigmatism, a plano-parallel plate 50 (astigmatism corrector) having a high index of refraction (e.g., approximately 1.93 for PBH-71 glass) can be used next to the multi-layer reflective polarizer as shown in FIG. 3C. However, the use of such an astigmatism corrector significantly increases the cost of the PBS.